1. Field of The Invention
The present invention relates to an apparatus for detecting a position of a cage of a hydraulic elevator with an accuracy of several millimeters through the whole service travel between a bottom floor and a top floor, particularly to an apparatus capable of specifying the position of a cage, which stops due to the power failure and sinks due to the discharge of oil from a hydraulic jack occurring during the power failure, for reopening of a normal operation for service after the recovery of power.
2. Description of The Related Art
A conventional hydraulic elevator has not been provided with a cage position detector using such a pulse generator as generates a pulse for every predetermined distance of the travel of an elevator cage. This is because, in a hydraulic elevator, the number of floors served by the elevator was generally not so large and further the fine position of a cage has not been required because of the simplicity of the speed control.
In addition thereto, there was a particular reason in a hydraulic elevator why a position detector of the type as mentioned above has not been employed. A hydraulic elevator generally has no braking device of such a type as a so called rope type elevator has, and a cage thereof is supported by a hydraulic jack which is filled up with pressure oil, during the cage stops. Therefore, if the cage stops for long time, it sinks little by little, since the pressure oil is gradually discharged from the hydraulic jack by the leakage of oil in various valves included in the hydraulic system. It is difficult to prevent the leakage of oil perfectly. Accordingly, in the normality of power, a hydraulic pump was operated to make the cage rise, when the amount of the sinkage thereof reached a predetermined value.
During the power failure, however, the pump can not be operated, so that the cage continues to sink gradually. Even if a position, at which a cage stops due to the power failure, is detected and memorized, an actual location at which the cage positions after a while is different from the position memorized. It was very difficult to make sure of a location where the cage positions at the time of the recovery of power. If the present position of a cage is lost, the elevator can no longer controlled normally.
To this end, every time the power failure occurs, the cage was traveled to a specific floor so that the location where the cage positions at the time of the recovery of power could be specified. This was done for the preparation of the reopening of a normal operation for service after the recovery of power. Namely, the elevator can always reopen the normal operation from the specific floor after the power recovers.
Further, in a hydraulic elevator, such a preparatory operation as mentioned above is possible by discharging pressure oil from a hydraulic jack by a manual control. Accordingly, the preparatory operation is a downward running, and therefore the specific floor was generally selected at a lowest floor.
For a rope type elevator, there have been developed a cage position detector which can cope with various problems accompanied by the power failure. Such a detector detects the traveling distance of the cage from the occurrence of the power failure to the actual stop of the cage. Once the cage has been stopped by a braking device, it never moves during the braking force is effective. Therefore, the detector is sufficient to be operable only during the very short time from the occurrence of the power failure to the actual stop of the cage. The thus detected position can be maintained by a little of backup power.
By the way, the application of hydraulic elevators is widened again and again. As a result, the number of floors to be served by a hydraulic elevator increases and a concern is being paid about the detection of the fine position of a cage. Then, it is considered to apply such a cage position detector as developed for a rope type elevator to a hydraulic elevator. However, in case such detector is applied to a hydraulic elevator, the detector must be maintained operable throughout the duration of the power failure, since, as mentioned above, the cage continues to sink during the whole duration of the power failure. To provide an auxiliary power for maintaining the detector operable throughout the duration of the power failure is very costly and therefore disadvantageous for an economy-oriented hydraulic elevator.